The present disclosure generally relates to shell molds for directional casting, and more particularly, to high thermal conductivity shell mold compositions that provide a high heat extraction rate.
In the manufacture of components, such as nickel based superalloy turbine blades and vanes for turbine engines, directional solidification (DS) investment casting techniques have been employed to produce columnar grain and single crystal casting microstructures having improved mechanical properties at the high temperatures encountered in the turbine section of the engine.
For directional solidification of superalloys, the solid-liquid interface applies a high thermal gradient to yield good cast microstructure. In order to provide a high thermal gradient, heat needs to be removed quickly from the solid casting.
Conventional investment shell molds for directional solidification of super alloys and other transition metal-based alloys are made of alumina or zircon particulates. Typical thermal conductivities of the conventional shell molds are about 1.42 W/m-K for alumina shells and 1.62 W/m-K for zircon shells. The typical shell thickness ranges between 8 to 12 millimeters depending on casting size and geometry. Heat extraction from solidifying casting in Liquid Metal Cooling (LMC) directional solidification is limited by shell heat conduction when the conventional shell molds are applied. Accordingly, there remains a need for molds with improved heat extraction through thermal conductivity increase or shell thickness reduction, or both.